Fiber-drawing method

Abstract

The method consists of increasing the flowrate of argon injected into the bottom of a fiber-drawing furnace when a weld between two sections of a preform is being drawn during the operation of drawing a fiber from a preform. This prevents the weld causing an excessive increase in the diameter of the resulting fiber and/or an excessive variation in the drawing speed.

1 . A method of drawing a preform, the method including: 
 drawing the end of the heated preform into a fiber; and    injecting at least a first gas in the vicinity of the heated part of the preform;    and being characterized by varying the flowrate at which the first gas is injected in the presence of an irregularity in the preform.    
     
     
         2 . The method of    claim 1   , wherein the irregularity consists of a weld between two sections of the preform.  
     
     
         3 . The method of    claim 1   , wherein the flowrate at which the first gas is injected is varied as a function of time in accordance with a predetermined curve.  
     
     
         4 . The method of    claim 1   , wherein the flowrate at which the first gas is injected is varied at a rate of less than 4 liters per minute per second, preferably at a rate of less than 1 liter per minute per second.  
     
     
         5 . The method of    claim 1   , wherein the first gas injected is argon and wherein the step of varying the injection flowrate includes: 
 increasing the flowrate of argon injected in the presence of an irregularity likely to increase the diameter of the fiber, other things being equal; or    reducing the flowrate of argon injected in the presence of an irregularity likely to reduce the diameter of the fiber, other things being equal.    
     
     
         6 . The method of    claim 5   , wherein the injection flowrate is varied by 10 l/min to 20 l/min or less.  
     
     
         7 . The method of    claim 1   , wherein at least a second gas is injected in the vicinity of the heated part of the preform and the flowrate at which the second gas is injected is varied in the opposite direction to the variation of the flowrate at which the first gas is injected.  
     
     
         8 . The method of    claim 7   , wherein the absolute value of the variation in the flowrate at which the second gas is injected is substantially equal to the absolute value of the variation of the flowrate at which the first gas is injected.  
     
     
         9 . The method of    claim 7   , wherein the second gas is helium.  
     
     
         10 . The method of    claim 1   , wherein the first gas injected is helium and wherein the step of varying the injection flowrate includes: 
 increasing the flowrate of helium injected in the presence of an irregularity likely to reduce the diameter of the fiber obtained, other things being equal; or    reducing the flowrate of helium injected in the presence of an irregularity likely to increase the diameter of the fiber obtained, other things being equal.    
     
     
         11 . The method of    claim 1   , wherein the step of varying the flowrate at which the first gas is injected has a duration in the range from 100 s to 350 s.  
     
     
         12 . The method of    claim 1   , including measuring the diameter of the fiber and wherein the flowrate at which the first gas is injected is varied in accordance with the measured diameter.  
     
     
         13 . The method of    claim 12   , wherein the flowrate at which the first gas is injected is varied if the measured diameter reaches a predefined value.  
     
     
         14 . The method of    claim 1   , including measuring the drawing speed and controlling the diameter of the fiber by action on the drawing speed and wherein the flowrate at which the first gas is injected is varied in accordance with the measured drawing speed.  
     
     
         15 . The method of    claim 14   , wherein the flowrate at which the first gas is injected is varied if the measured drawing speed reaches a predefined value.  
     
     
         16 . The method of    claim 1   , characterized by multivariable control of the drawing tension and the drawing speed by operating on the rate of descent of the preform and/or on the heating power applied to the end of the preform.